I had the 50/1.8 and I rather have the 40/2.8 due to the optical quality.
Same here, the 'lil 40 is much better in every way.
The Plastic Fantastic is over a stop faster and a bit longer and thus can get a shallower depth of field.
The extra stop is nice for low light, too, but the insane high ISO abilities of modern cameras make that advantage of wide apertures much less important.
If I had to choose between the Plastic Fantastic and the Shorty McForty, I'd pick the Shorty McForty in a heartbeat, though. The Plastic Fantastic wouldn't be my go-to lens for wide aperture work, so that one area where it narrowly beats the Shorty McForty is irrelevant to me.